In a prior art method of fabricating AlGaInP/GaInP series lasers, efforts are being made to fabricate a semiconductor laser which has a window structure in the proximity of the laser resonator facet, that is, a structure whose energy band gap is widened such that the absorption of the laser light at the laser resonator facets is reduced and the COD (Catastrophic Optical Damage) level is raised, thereby obtaining a semiconductor laser having a high power output and along lifetime. This window structure is obtained, for example, by disordering a natural superlattice of an active layer by an impurity diffusion in the proximity of the laser resonator facet, thereby widening an energy band gap in the proximity of the laser resonator facet, or by growing crystals which have a wide energy band gap at the laser resonator facet.
FIG. 27(a) is a perspective view showing a structure of a prior art AlGaInP/GaInP series semiconductor laser which has a window structure, and FIG. 27(b) is a cross-sectional view taken along the line 27--27. In the figures, reference numeral 300 designates a semiconductor laser, reference numeral 12 designates an n type GaAs substrate, reference numeral 11 designates an n type AlGaInP cladding layer, reference numeral 10 designates a GaInP active layer, reference numeral 55 designates a window structure, reference numerals 9a and 9b designate p type AlGaInP cladding layers, reference numeral 41 designates a p type GaInP etching stopper layer, reference numeral 8 designates a p type GaInP band discontinuity layer, reference numeral 7a designates a p type GaAs cap layer, reference numeral 7b designates a p type GaAs contact layer, reference numeral 6a designates an n side electrode, reference numeral 6b designates a p side electrode, reference numeral 40 designates an n type GaAs current blocking layer, and reference numeral 3b designates a laser resonator facet.
Furthermore, FIGS. 28(a)-28(d) are perspective views illustrating a prior art method of fabricating a semiconductor laser. In the figures, the same reference numerals used in FIGS. 27(a) and 27(b) designate the same or corresponding parts, and reference numerals 50 and 51 designate a first insulating film and second insulating film, respectively, comprising materials such as SiN or SION.
The fabricating method will be described. First, the n type AlGaInP cladding layer 11, the GaInP active layer 10, the p type AlGaInP cladding layer 9a, the p type GaInP etching stopper layer 41, the p type AlGaInP cladding layer 9b, the p type GaInP band discontinuity layer 8, and the p type GaAs cap layer 7a are epitaxially grown in this order on the n type GaAs substrate 12 by MOCVD (Metal Organic Chemical Vapor Deposition). These epitaxial layers are grown at the temperature of about 650.degree. C. so that the natural superlattice is formed in the active layer 10.
Next, the first insulating film 50 is formed by CVD (Chemical Vapor Deposition) or sputtering on the semiconductor laminated layer structure prepared by MOCVD. Then, a resist is deposited on the insulating film 50, and a portion of the insulating film 50 which becomes the laser resonator facet proximity region is removed by photolithography and dry-etching methods such as RIE (Reactive Ion Etching) or wet-etching with a solution such as buffered hydrofluoric acid. When the length in the resonator length direction of the semiconductor laser 300 is, for example, 650 .mu.m, the portion of the first insulating film 50 from the laser resonator facet to more than 20 .mu.m in the resonator length direction is removed. Next, after the p type GaAs cap layer 7a is selectively etched with the first insulating film 50 as a mask using a tartaric acid based etchant, a ZnO film (not shown in the figure) is grown by CVD or sputtering. Then, Zn is diffused through an opening of the first insulating film 50 and the cap layer 7a by a heat treatment at 550.degree. C. for about one hour until Zn reaches the active layer. This Zn diffusion disorders the natural superlattice of the active layer 10. Consequently, the band gap energy of the portion of the active layer which becomes the laser resonator facet proximity region becomes greater than the band gap energy of the remaining portion of the active layer, thereby forming a window structure 55 in the laser resonator facet proximity region. The ZnO film is removed later (FIG. 28(b)).
Next, the second insulating film 51 is formed on the semiconductor laminated layer structure and patterned into a stripe shape extending in the resonator length direction by photolithography or the like. Then, using this stripe-shaped insulating film 51 as a mask, the p type GaAs cap layer 7a is selectively etched using a tartaric acid based etchant. Next, the p type GaInP band discontinuity layer 8 is etched and removed with a hydrochloric acid based etchant, and the p type AlGaInP cladding layer 9b is etched and removed with sulfuric acid based etchant until the etching front reaches the p type GaInP etching stopper layer 14, thereby forming a ridge structure.
Next, using the second insulating film 51 as a mask, the n type GaAs current blocking layer 40 is selectively grown so as to bury the ridge, and after the first and the second insulating films 51 and 52 are removed, the p type GaAs contact layer 7b is formed on the semiconductor laminated layer structure (FIG. 28(d)). Then, the n side electrode 6b is formed on the p type GaAs contact layer 7b and the p side electrode 6a is formed on the rear surface side of the n type GaAs substrate 12. Finally, by cleaving the semiconductor laminated layer structure at a location where the window structure 55 is formed by diffusing Zn laser resonator facet, the laser resonator facet 3b is formed and the semiconductor laser 300 illustrated in FIG. 27(a) is obtained.
In the prior art semiconductor laser 300 shown in FIGS. 27(a) and 27(b), a dopant impurity such as Zn is diffused in the proximity of the laser resonator facet of the active layer 10 where the natural superlattice is formed so that the natural superlattice in the region diffused with the Zn is disordered and becomes of a mixed crystal having a homogeneous composition, and the energy band gap of this region becomes larger than that of other regions of the active layer 10, thereby making this region diffused with the Zn a window structure 55 where the absorption of light does not take place. In the prior art semiconductor laser 300 having such a window structure at the laser resonator facet 3b, laser light emitted in the active layer 10 is not absorbed at this window structure and catastrophic optical damage (COD) can be avoided.
However, in the prior art method of fabricating a semiconductor laser, a Zn diffusion process is required in order to form a window structure by disordering the active layer in the proximity of the laser resonator facet. This diffusion process is hard to control and it is possible that too much Zn may be accidentally diffused in the proximity of the resonator facet. If too much Zn is diffused, the electrical resistance of the Zn diffused region decreases due to Zn, and leakage current which does not contribute to the laser oscillation is generated in the proximity of the laser resonator facet, i.e., at the window structure 55. This results in an increased threshold current or an increased operation current, and a semiconductor laser having desired characteristics cannot be obtained repeatedly.
FIG. 29(a) is a perspective view showing a structure of another prior art semiconductor laser and FIGS. 29(b) and 29(c) are cross-sectional views taken along the lines 29b--29b and 29c--29c in FIG. 29(a), respectively. This prior art semiconductor laser has an AlGaInP layer whose energy band gap is greater than the energy band gap of the active layer 10 on the laser resonator facet. In FIGS. 29(a), 29(b), and 29(c), the same reference numerals used in FIGS. 27(a) and 27(b) designate the same or corresponding parts. Reference numeral 400 designates a semiconductor laser, reference numeral 3c designates a laser resonator facet, and reference numeral 52 designates the AlGaInP window structure layer.
A method of fabricating this prior art semiconductor laser will be described with reference to FIGS. 29(a)-29(c). First, the n type AlGaInP cladding layer 11, the GaInP active layer 10, the p type AlGaInP cladding layer 9a, the p type GaInP etching stopper layer 41, the p type AlGaInP cladding layer 9b, the p type GaInP band discontinuity layer 8, and the p type GaAs cap layer 7a are epitaxially grown in this order on the n type GaAs substrate 12 by MOCVD. Then, an insulating film (not shown in the figures) comprising materials such as SiN or SiON is formed by CVD or sputtering, and a stripe mask (not shown in the figures) extending in the laser resonator length direction is formed using photolithography techniques or the like. Next, using this stripe mask, the p type GaAs cap layer 7 is selectively etched with a tartaric acid based etchant. Then, the p type GaInP band discontinuity layer 8 is etched with a hydrrochloric acid based etchant and the p type AlGaInP cladding layer 9 is etched with a sulfuric acid based etchant until the etching front reaches the p type GaInP etching stopper layer 41, thereby producing a ridge stripe structure. Then, after forming the n type GaAs current blocking layer 40 so as to bury the ridge, the insulating film is removed, the p type GaAs contact layer 7 is formed on the entire surface, and the p side electrode 6b is formed on the p type GaAs contact layer 7 and the n side electrode 6a is formed on the rear surface side of the n type GaAs substrate 12. Then the substrate 12 is cleaved to form the laser resonator facet 3c, and finally the window structure layer 52 comprising, for example, AlGaInP, whose energy band gap is large enough so that the laser light is not absorbed is grown on the laser resonator facet 3c, thereby obtaining the semiconductor laser shown in FIGS. 29(a)-29(c).
Also in the semiconductor laser 400 as the other prior art semiconductor laser, by disposing the window structure layer 52 whose energy band gap is larger than that of the active layer 10 on the laser resonator facet 3c, the laser light in the waveguide of the active layer 10 is not absorbed at this window structure layer 52 and, as a result, catastrophic optical damage at the laser resonator facet 3c can be avoided.
However, although, in a method of fabricating a semiconductor laser such as this, it is necessary to epitaxially grow the window structure layer comprising AlGaInP or the like at the laser resonator facet after the cleavage, since semiconductor lasers after the cleavages are quite delicate, their handling is difficult and there is a possibility that the laser resonator facet may be scratched during handling, thereby decreasing yield. Furthermore, since it is necessary to conduct the epitaxial growth after the cleavage, the fabricating process becomes complicated and productivity is decreased.
As described above, in the prior art method of fabricating a semiconductor laser, by diffusing Zn in the proximity of the laser resonator facet of the active layer and disordering the natural superlattice in the proximity of the laser resonator facet, the band gap in the proximity of the laser resonator facet of the active layer is widened so that the laser light is not absorbed in the proximity of the laser resonator facet, thereby forming the window structure and avoiding catastrophic optical damage. However, since it is difficult to control the diffusion of Zn, there is a possibility, as described above, that too much Zn is diffused in the proximity of the laser resonator facet and leakage current which does not contribute to the laser oscillation is generated in the proximity of the laser resonator facet, resulting in an increased threshold current or an increased operation current, and a semiconductor laser having desired characteristics can not be obtained repeatedly.
In the other prior art method of fabricating a semiconductor laser, although the COD is avoided by disposing the window structure layer having an energy band gap larger than the energy band gap of the active layer on the laser resonator facet, since it is necessary to epitaxially grow the window structure layer on the laser resonator facet of the quite delicate semiconductor laser after the cleavage, its handling becomes difficult, and there is a possibility that the yield is decreased or the productivity is decreased due to the fabricating process which becomes complicated.